Two-dimensional channel bonding in a hybrid CDMA/FDMA fixed wireless access system to provide finely variable rate channels

ABSTRACT

A communications system employs the use of both synchronous CDMA and FDMA to provide a variable bandwidth waveform with multiple bonded transmitters and receivers that are agile in both frequency and PN code to permit a variable bandwidth and variable rate multiple access system. In a first aspect the teachings provide the use of both CDMA and FDMA together to enable an improved concentration efficiency by making a larger pool of bandwidth available to each user. In a second aspect these teachings enable channel bonding across both code space and frequency space, thus making the system capable of operating within a variable (not necessarily contiguous) bandwidth and at a finely variable rate.

CLAIM OF PRIORITY FROM COPENDING PROVISIONAL PATENT APPLICATION

This patent application claims priority from U.S. Provisional PatentApplication No.: 60/243,972, filed on Oct. 27, 2000, the disclosure ofwhich is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

This invention relates generally to wireless communications systems andmethods, and relates in particular to hybrid Code Division MultipleAccess (CDMA)/Frequency Division Multiple Access (FDMA) systems having aplurality of different possible data transmission rates.

BACKGROUND OF THE INVENTION

Most cellular and fixed wireless access air-interfaces use either CDMA,FDMA or TDMA to provide logical channels to users.

Typically, CDMA and FDMA systems have one modulator and one demodulatorin a subscriber unit, and a bank of modulators and demodulators in thebase station. One link is typically established between each activesubscriber unit and the base station. Thus, the modulators anddemodulators typically communicate with each other using one pseudonoise(PN) code on one frequency at a time. In a CDMA application, it ispossible that these channels may be variable rate data channels and usevariable rate PN codes, such as those described in U.S. Pat. No.6,091,760, but these variable rate channels will typically be limited topower-of-two multiples of a basic rate. For example, if the basic ratechannel were 32 kbps, then the possible rates achievable will be 64,128, 256, 512, 1024, 2048 kbps, etc. Thus, if it is desirable to operatethe link at 1050 kbps, then the next higher rate (2048 kbps) must beused, which is wasteful of system bandwidth.

Transmitting and receiving on multiple links in parallel to achieveadditional rates is known in CDMA systems. For example, reference can behad to U.S. Pat. Nos. 5,373,502, 5,442,625 and 5,166,951 in this regard.

SUMMARY OF THE INVENTION

A communications system employs the use of both CDMA and FDMA to providea variable bandwidth waveform with multiple bonded transmitters andreceivers that are agile in both frequency and PN code to permit avariable bandwidth and variable rate multiple access system.

In a first aspect the teachings provide the use of both CDMA and FDMAtogether to enable an improved concentration efficiency by making alarger pool of bandwidth available to each user. In a second aspectthese teachings enable channel bonding across both code space andfrequency space, thus making the system capable of operating within avariable (not necessarily contiguous) bandwidth and at a finely variabledata rate.

A synchronous Code Division Multiple Access S-CDMA and FrequencyDivision Multiple Access FDMA communications system in accordance withthese teachings includes a base site having a transmitter fortransmitting a waveform, where the transmitter includes a plurality offrequency agile and PN code agile data modulators having an outputcoupled to a radio channel. The system further includes a subscriberunit having a receiver for receiving the transmitted waveform from theradio channel, where the receiver includes a plurality of frequencyagile and PN code agile data demodulators.

There may be N modulators and N demodulators, each operable forcommunicating at data rates that are power of two multiples of a basicrate on a plurality of frequency subchannels within a channel. The Nmodulators and N demodulators operate with power of two multiples of thebasic rate from a minimum rate to a maximum rate at a granularity thatis an integer multiple of the basic rate.

Statistical concentration is achieved by providing the system with YMbps of aggregate capacity allocatable to X users simultaneously atrates of Y/X Mbps each, and by operating the N modulators and Ndemodulators to any one of Z frequency subchannels. In this case theuseable bandwidth is Z times the Y Mbps bandwidth of any one channel,and Z*X users are supported simultaneously at rates of Y/X Mbps.

A bandwidth of any one subchannel is X MHz, and at least some of theplurality of modulators and demodulators are tuned to different ones ofcontiguous or non-contiguous X MHz sub-channels within a Y MHz channel,where Y>X. For example, X=3.5 and Y=14.

In the presently preferred, but not limiting embodiment the input datato the plurality of modulators is a punctured convolutional code, suchas a rate 1/2, constraint length 7 code that is punctured to increasethe rate of the code. The puncturing rate can be made adaptive tomitigate fading conditions. The output of the plurality of modulatorscan be coupled to the radio channel through an end-to-end raised-cosineNyquist pulse shape filter.

BRIEF DESCRIPTION OF THE DRAWINGS

The above set forth and other features of the invention are made moreapparent in the ensuing Detailed Description of the Invention when readin conjunction with the attached Drawings, wherein:

FIG. 1 is simplified block diagram of a wireless access reference modelthat pertains to the teachings of this invention;

FIG. 2 is block diagram of a physical (PHY) system reference modelshowing a major data flow path;

FIG. 3 shows an Error Control Coding (ECC) and scrambling technique forsingle CDMA channel;

FIG. 4 is a Table illustrating exemplary parameters for a 3.5 MHz RFchannelization;

FIG. 5 is a Table depicting an aggregate capacity and modulation factorsversus modulation type and array size; and

FIG. 6 shows the use of multiple modulators and multiple demodulators inaccordance with an aspect of these teachings.

DETAILED DESCRIPTION OF THE INVENTION

Disclosed herein is a PHY system intended for IEEE 802.16.3 and relatedstandards, although those having skill in the art should realize thatvarious aspects of these teachings have wider applicability.

The technique is based on a hybrid synchronous DS-CDMA (S-CDMA) and FDMAscheme using quadrature amplitude modulation (QAM) and trellis coding.For a general background and benefits of S-CDMA with trellis-coded QAMone may refer to R. De Gaudenzi, C. Elia and R. Viola, “BandlimitedQuasi-Synchronous CDMA: A Novel Satellite Access Technique for Mobileand Personal Communication Systems,” IEEE Journal on Selected Areas inCommunications, Vol. 10, No. 2, February 1992, pp. 328–343, and to R. DeGaudenzi and F. Gianneti, “Analysis and Performance Evaluation ofSynchronous Trellis-Coded CDMA for Satellite Applications,” IEEETransactions on Communications, Vol. 43, No. 2/3/4, February/March/April1995, pp. 1400–1409.

The ensuing description focuses on a frequency division duplexing (FDD)mode. While a time division duplexing (TDD) mode is also within thescope of these teachings, the TDD mode is not discussed further.

What follows is an overview of the PHY teachings in accordance with thisinvention.

The system provides synchronous direct-sequence code division multipleaccess (DS-CDMA) for both upstream and downstream transmissions. Thesystem further provides spread RF channel bandwidths from 1.75–7 MHz,depending on target frequency band, and a constant chip rate from 1–6Mcps (Million chips per second)within each RF sub-channel with commonI-Q spreading. The chip rate depends on channelization of interest (e.g.3.5 MHz or 6 MHz). The system features orthogonal, variable-lengthspreading codes using Walsh-Hadamard designs with spread factors (SF) of1, 2, 4, 8, 16, 32, 64 and 128 chips/symbol being supported, and alsofeatures unique spreading code sets for adjacent, same-frequencycells/sectors. Upstream and downstream power control and upstream linktiming control are provided, as are single CDMA channel data rates from32 kbps up to 16 Mbps depending on SF (spreading factor) and chip rate.In the preferred system S-CDMA channel aggregation is provided for thehighest data rates.

Furthermore, in the presently preferred embodiment FDMA is employed forlarge bandwidth allocations with S-CDMA in each FDMA sub-channel, andS-CDMA/FDMA channel aggregation is used for the higher data rates. Code,frequency and/or time division multiplexing is employed for bothupstream and downstream transmissions. Frequency division duplex (FDD)or time division duplex (TDD) can be employed, although as stated abovethe TDD mode of operation is not described further. The system featurescoherent QPSK and 16-QAM modulation with optional support for 64-QAM.End-to-end raised-cosine Nyquist pulse shape filtering is employed, asis adaptive coding, using high-rate punctured, convolutional coding(K=7) and/or Turbo coding (rates of 4/5, 5/6 and 7/8 are typical). Datarandomization using spreading code sequences is employed, as is linearequalization in the downstream with possible transmit pre-equalizationfor the upstream. Also featured is the use of space division multipleaccess (SDMA) using adaptive beamforming antenna arrays (1 to 16elements possible) at the base station.

FIG. 1 shows the wireless access reference model per the IEEE 802.16.3FRD (see IEEE 802.16.3-00/02r4, “Functional Requirements for the802.16.3 Interoperability Standard.”). Within this model, the PHYtechnique in accordance with these teachings provides access between oneor more subscriber stations (SS) 10 and base stations 11 to support theuser equipment 12 and core network 14 interface requirements. Anoptional repeater 16 may be deployed.

In FIG. 2, the PHY reference model is shown. This reference model isuseful in discussing the various aspects of the PHY technique. As isapparent, the SS 10 and BS transmission and reception equipment may besymmetrical. In a transmitter 20 of the BS 11 or the SS 10 there is anError Control Coding (ECC) encoder 22 for incoming data, followed by ascrambling block 24, a modulation block 26 and a pulseshaping/pre-equalization block 28. In a receiver 30 of the BS 11 or theSS 10 there is a matched filter/equalization block 32, a demodulationblock 34, a descrambling block 36 and an ECC decoder 38. These variouscomponents are discussed in further detail below.

The PHY interfaces with the Media Access Control (MAC) layer, carryingMAC packets and enabling MAC functions based on Quality of Service (QoS)requirements and Service Level Agreements (SLAs). As a S-CDMA system,the PHY interacts with the MAC for purposes of power and timing control.Both power and timing control originate from the BS 11, with feedbackfrom the SS 10 needed for forward link power control. The PHY alsointeracts with the MAC for link adaptation (e.g. bandwidth allocationand SLAs), allowing adaptation of modulation formats, coding, datamultiplexing, etc.

With regard to frequency bands and RF channel bandwidths, the primaryfrequency bands of interest for the PHY include the ETSI frequency bandsfrom 1–3 GHz and 3–11 GHz as described in ETSI EN 301 055, Fixed RadioSystems; Point-to-multipoint equipment; Direct Sequence Code DivisionMultiple Access (DS-CDMA); Point-to-point digital radio in frequencybands in the range 1 GHz to 3 GHz, and in ETSI EN 301 124, Transmissionand Multiplexing (TM); Digital Radio Relay Systems (DRRS); DirectSequence Code Division Multiple Access (DS-CDMA) point-to-multipointDRRS in frequency bands in the range 3 GHz to 11 GHz, as well as withthe MMDS/MDS (digital TV) frequency bands. In ETSI EN 301 124, the radiospecifications for DS-CDMA systems in the fixed frequency bands around1.5, 2.2, 2.4 and 2.6 GHz are given, allowing channelizations of 3.5, 7,10.5 and 14 MHz. Here, the Frequency Division Duplex (FDD) separation isspecific to the center frequency and ranges from 54 to 175 MHz. In ETSIEN 301 124, Transmission and Multiplexing (TM); Digital Radio RelaySystems (DRRS); Direct Sequence Code Division Multiple Access (DS-CDMA)point-to-multipoint DRRS in frequency bands in the range 3 GHz to 11GHz., the radio characteristics of DS-CDMA systems with fixed frequencybands centered around 3.5, 3.7 and 10.2 GHz are specified, allowingchannelizations of 3.5, 7, 14, 5, 10 and 15 MHz. Here, FDD separation isfrequency band dependant and ranges from 50 to 200 MHz. Also of interestto these teachings are the MMDS/ITSF frequency bands between 2.5 and 2.7GHz with 6 MHz channelizations.

With regard to multiple access, duplexing and multiplexing, theteachings herein provide a frequency division duplex (FDD) PHY using ahybrid S-CDMA/FDMA multiple access scheme with SDMA for increasedspectral efficiency. In this approach, a FDMA sub-channel has an RFchannel bandwidth from 1.75 to 7 MHz. The choice of FDMA sub-channel RFchannel bandwidth is dependent on the frequency band of interest, with3.5 MHz and 6 MHz being typical per the IEEE 802.16.3 FRD. Within eachFDMA sub-channel, S-CDMA is used with those users transmitting in theupstream and downstream using a constant chipping rate from 1 to 6Mchips/second. While TDD could be used in a single RF sub-channel, thisdiscussion is focused on the FDD mode of operation. Here, FDMAsub-channel(s) are used in the downstream while at least one FDMAsub-channel is required for the upstream. The approach is flexible toasymmetric data traffic, allowing more downstream FDMA sub-channels thanupstream FDMA sub-channels when traffic patterns and frequencyallocation warrant. Based on existing frequency bands, typicalupstream/downstream FDMA channel separation range from 50 to 200 MHz.

Turning now to the Synchronous DS-CDMA (S-CDMA) aspects of theseteachings, within each FDMA sub-channel, S-CDMA is used in both theupstream and the downstream directions. The chipping rate is constantfor all SS with rates ranging from 1 to 6 Mchips/second depending on theFDMA RF channel bandwidth. Common I-Q spreading is performed usingorthogonal, variable-length spreading codes based on Walsh-Hadamarddesigns with spread factors ranging from 1 up to 128 chips per symbol(see, for example, E. Dinan and G. Jabbari, “Spreading Codes for DirectSequence CDMA and Wideband CDMA Cellular Networks,” IEEE CommunicationsMagazine, September 1998, pp.48–54. For multi-cell deployments with lowfrequency reuse, unique spreading code sets are used in adjacent cellsto minimize interference.

It should be noted that an aspect of these teachings is a symmetricwaveform within each FDMA sub-channel, where both the upstream anddownstream utilize the same chipping rate (and RF channel bandwidth),spreading code sets, modulation, channel coding, pulse shape filtering,etc.

Referring now to Code and Time Division Multiplexing and channelaggregation, with a hybrid S-CDMA/FDMA system it is possible tomultiplex data over codes and frequency sub-channels. Furthermore, for agiven code or frequency channel, time division multiplexing could alsobe employed. In the preferred approach, the following multiplexingscheme is employed.

For the downstream transmission with a single FDMA sub-channel, thechannel bandwidth (i.e. capacity measured in bits/second) is partitionedinto a single TDM pipe and multiple CDM pipes. The TDM pipe may becreated via the aggregation of multiple S-CDMA channels. The purpose ofthis partition is based on the desire to provide Quality of Service(QoS). Within the bandwidth partition, the TDM pipe would be used forbest effort service (BES) and for some assured forwarding (AF) traffic.The CDM channels would be used for expedited forwarding (EF) services,such as VoIP connections or other stream applications, where the datarate of the CDM channel is matched to the bandwidth requirement of theservice.

The downlink could be configured as a single TDM pipe. In this case atime slot assignment may be employed for bandwidth reservation, withtypical slot sizes ranging from 4–16 ms in length. While a pure TDMdownlink is possible in this approach, it is preferred instead to employa mixed TDM/CDM approach. This is so because long packets can inducejitter into EF services in a pure TDM link. Having CDMA channels (singleor aggregated) dedicated to a single EF service (or user) reduces jitterwithout the need for packet fragmentation and reassembly. Furthermore,these essentially “circuit-switched” CDM channels would enable bettersupport of legacy circuit-switched voice communications equipment andpublic switched telephone networks.

For the upstream, the preferred embodiment employs a similar partitionof TDM/CMD channels. The TDM channel(s) would be used for random access,using a slotted-Aloha protocol. In keeping with a symmetric waveform,recommended burst lengths are on the order of the slot times for thedownlink, ranging from 4–16 ms. Multi-slot bursts are possible. The BS11 monitors bursts from the SS 10 and allocates CDMA channels to SSsupon recognition of impending bandwidth requirements or based on servicelevel agreements (SLAs). As an example, a BS 11 recognizing theinitiation of a VoIP connection could move the transmission to adedicated CDMA channel with a channel bandwidth of 32 kbps.

When multiple FDMA sub-channels are present in the upstream ordownstream directions, similar partitioning could be used. Here,additional bandwidth exists which implies that more channel aggregationis possible. With a single TDM channel, data may be multiplexed acrossCDMA codes and across frequency sub-channels.

With regard now to Space Division Multiple Access (SDMA) extensions, afurther aspect of this multiple access scheme involves the use of SDMAusing adaptive beamforming antennas. Reference can be made to J. Libertiand T. Rappaport, Smart Antennas for Wireless CDMA, Prentice-Hall PTR,Upper Saddle River, N.J., 1997, for details of beamforming with CDMAsystems.

In accordance with the teachings herein there is provided an adaptiveantenna array at the BS 11, with fixed beam SS antennas. In thisapproach, S-CDMA/FDMA channels can be directed at individual SSs. Theisolation provided by the beamforming allows the CDMA spreading codes tobe reused within the same cell, greatly increasing spectral efficiency.Beamforming is best suited to CDM rather than TDM channels. In thedownstream, TDM would employ beamforming on a per slot or burst basis,increasing complexity. In the upstream, beamforming would be difficultsince the BS 11 would need to anticipate transmission from the SS inorder to form the beams appropriately. In either case, reuse of CDMAspreading codes in a TDM-only environment would be difficult. With CDM,however, the BS 11 may allocate bandwidth (i.e. CDMA channels) to SS 10based on need, or on SLAs. Once allocated, the BS 11 forms a beam to theSS 10 to maximize signal-to-interference ratios. Once the beam isformed, the BS 11 may allocate the same CDMA channel to one or moreother SSs in the cell. It is theoretically possible for the spectralefficiency of the cell to scale linearly with the number of antennas inthe BS array.

SDMA greatly favors the approach of “fast circuit-switching” over pure,TDM packet-switching in a CDMA environment. By “fast circuit-switching”,what is implied is that packet data services are handled using dedicatedconnections, which are allocated and terminated based on bandwidthrequirements and/or SLAs. An important consideration when providingeffective packet-services using this approach lies in the ability of theBS 11 to rapidly determine bandwidth needs, and to both allocate andterminate connections rapidly. With fast channel allocation andtermination, SDMA combined with the low frequency reuse offered byS-CDMA is a preferred option, in terms of spectral efficiency, for FWAapplications.

A discussion is now made of waveform specifications. The waveformincludes the channel coding 22, scrambling 24, modulation 26 and pulseshaping and equalization functions 28 of the air interface, as depictedin FIG. 2. Also included are waveform control functions, including powerand timing control. In the presently preferred PHY, each CDMA channel(i.e. spreading code) uses a common waveform, with the spreading factordictating the data rate of the channel.

With regard to the Error Control Coding (ECC) function 22 of FIG. 2, theECC is preferably high-rate and adaptive. High rate codes are used tomaximize the spectral efficiency of BWA systems using S-CDMA systemsthat are code-limited. In code-limited systems, the capacity is limitedby the code set cardinality rather than the level of the multi-userinterference. Adaptive coding is preferred in order to improveperformance in multipath fading environments. For the coding options,and referring as well to FIG. 3, the baseline code is preferably apunctured convolutional code (CC). The constituent code may be theindustry standard, rate 1/2, constraint length 7 code with generator(133/171)₈. Puncturing is used to increase the rate of the code, withrates of 3/4, 4/5, 5/6 or 7/8 supported using optimum free distancepuncturing patterns. The puncturing rate of the code may be adaptive tomitigate fading conditions. For decoding (block 38 of FIG. 2), a Viterbidecoder is preferred. Reference in this regard can be made again to theabove-noted publication R. De Gaudenzi and F. Gianneti, “Analysis andPerformance Evaluation of Synchronous Trellis-Coded CDMA for SatelliteApplications,” IEEE Transactions on Communications, Vol. 43, No. 2/3/4,February/March/April 1995, pp. 1400–1409, for an analysis oftrellis-coded S-CDMA.

Turbo coding, including block turbo codes and traditional parallel andserial concatenated convolutional codes, are preferably supported as anoption at the rates suggested above. In FIG. 3, the CC/Turbo coding isperformed in block 22A, the puncturing in block 22B, and the scramblingcan be performed using an XOR 24A that receives a randomizing code.

Each CDMA channel is preferably coded independently. Independent codingof CDMA channels furthers the symmetry of the upstream and downstreamwaveform and enables a similar time-slot structure on each CDMA channel.The upstream and downstream waveform symmetry aids in cost reduction, asthe SS 10 and BS 11 baseband hardware can be identical. The independentcoding of each S-CDMA/FDMA channel is an important distinction betweenthis approach and other multi-carrier CDMA schemes.

Randomization is preferably implemented on the coded bit stream. Ratherthan using a traditional randomizing circuit, it is preferred, as shownin FIG. 3, to use randomizing codes derived from the spreading sequencesused by the transmitting station. Using the spreading codes allowsdifferent randomizing sequences to be used by different users, providingmore robust randomization and eliminating problems with inter-usercorrelated data due to periodic sequences transmitted (e.g. preambles).Since the receiving station has knowledge of the spreading codes,de-randomization is trivial. Randomization may be disabled on a perchannel or per symbol basis. FIG. 3 thus depicts the preferred channelcoding and scrambling method for a single CDMA channel.

With regard to the modulation block 26, both coherent QPSK and square16-QAM modulation formats are preferably supported, with optionalsupport for square 64-QAM. Using a binary channel coding technique,Gray-mapping is used for constellation bit-labeling to achieve optimumdecoded performance. This combined coding and modulation scheme allowssimple Viterbi decoding hardware designed for binary codes to be used.Differential detection for all modulation formats may be supported as anoption. Depending on the channel coding, waveform spectral efficienciesfrom 1 to 6 information bits/symbol are realized.

The modulation format utilized is preferably adaptive based on thechannel conditions and bandwidth requirements. Both upstream anddownstream links are achievable using QPSK waveform provided adequateSNR. In environments with higher SNR, up and downstream links mayutilize 16-QAM and /or 64-QAM modulation formats for increased capacityand spectral efficiency. The allowable modulation format depends on thechannel conditions and the channel coding being employed on the link.

In the preferred embodiment end-to-end raised-cosine Nyquist pulseshaping is applied by block 28 of FIG. 2, using a minimum roll-offfactor of 0.25. Pulse shape filtering is designed to meet relevantspectral masks, mitigate inter-symbol interference (ISI) and adjacentFDMA channel interference.

To mitigate multipath fading, a linear equalizer 32 is preferred for thedownstream.

Equalizer training may be accomplished using a preamble, withdecision-direction used following initial training. With S-CDMA,equalizing the aggregate signal in the downlink effectively equalizesall CDMA channels. Multipath delay spread of less than 3 μs is expectedfor Non-Line Of Sight (NLOS) deployments using narrow-beam (10–20°)subscriber station 10 antennas (see, for example, J. Porter and J.Thweat, “Microwave Propagation Characteristics in the MMDS FrequencyBand,” Proceedings of IEEE International Conf. On Communications (ICC)2000, New Orleans, La., USA, June 2000, and V. Erceg, et al, “A Modelfor the Multipath Delay Profile of Fixed Wireless Channels,” IEEEJournal on Selected Areas in Communications (JSAC), Vol. 17, No. 3,March 1999, pp. 399–410.

The low delay spread allows simple, linear equalizers with 8–16 tapsthat effectively equalize most channels. For the upstream,pre-equalization may be used as an option, but requires feedback fromthe subscriber station 10 due to frequency division duplexing.

Timing control is required for S-CDMA. In the downstream, timing controlis trivial. However, in the upstream timing control is under thedirection of the BS 11. Timing control results in reduced in-cellinterference levels. While infinite in-cell signal to interferenceratios are theoretically possible, timing errors and reduction incode-orthogonality from pulse shape filtering allows realistic signal toin-cell interference ratios from 30–40 dB. In asynchronous DS-CDMA(A-CDMA) systems, higher in-cell interference levels exist, lessout-of-cell interference can be tolerated and higher frequency reuse isneeded to mitigate out-of-cell interference(see, for example, T.Rappaport, Wireless Communications: Principles and Practice,Prentice-Hall PTR, Upper Saddle River, N.J., 1996, pp. 425–431. Theability of timing-control to limit in-cell interference is an importantaspect of achieving a frequency reuse of one in a S-CDMA system.

Power control is also required for S-CDMA systems. Power control acts tomitigate in-cell and out-of-cell interference while also ensuringappropriate signal levels at the SS 10 or the BS 11 to meet bit errorrate (BER) requirements. For a SS 10 close to the BS 11, lesstransmitted power is required, while for a distant SS 10, more transmitpower is required in both the up and downstream. As with timing control,power control is an important aspect of achieving a frequency reuse ofone.

Turning now to a discussion of capacity, spectral efficiency and datarates, for a single, spread FDMA channel, the presently preferred S-CDMAwaveform is capable of providing channel bandwidths from 1 to 16 Mbps.Using variable-length spreading codes, each CDMA channel can beconfigured to operate from 32 kbps (SF=128) to 16 Mbps (SF=1), withrates depending on the modulation, coding and RF channel bandwidths.With S-CDMA channel aggregation, high data rates are possible withoutrequiring a SF of one. In general, the use of S-CDMA along with thepresently preferred interference mitigation techniques enable the systemto be code-limited. Note, mobile cellular A-CDMA systems are alwaysinterference-limited, resulting in lower spectral efficiency. Recallalso that in code-limited systems, the capacity is limited by the codeset cardinality rather than the level of the multi-user interference. Ina code-limited environment, the communications channel bandwidth of thesystem is equal to the communications channel bandwidth of the waveform,assuming a SF of one. In the Table shown in FIG. 4 sample parameters areshown for a hypothetical system using different coded modulation schemesand assuming a code-limited DS-CDMA environment. The Table of FIG. 4illustrates potential performance assuming a single 3.5 MHz channel inboth the upstream and downstream. The numbers reported apply to both theupstream and downstream directions, meaning that upwards of 24 Mbps fullduplex is possible (12 Mbps upstream and 12 Mbps downstream). Withadditional FDMA RF channels or large RF channels (e.g. 6 MHz),additional communication bandwidth is possible with the same modulationfactors from the Table. As an example, allocation of 14 MHz could beserviced using 4 FDMA RF channels with the parameters described in theTable of FIG. 4. At 14 MHz, peak data rates to a given SS 10 of up to 48Mbps are achievable, with per-CDMA channel data rates scaling up from 32kbps. The channel aggregation method in accordance with these teachingsis very flexible in servicing symmetric versus asymmetric traffic, aswell as for providing reserved bandwidth for QoS and SLA support.

With regard to multi-cell performance, to this point both the capacityand spectral efficiency have been discussed in the context of a single,isolated cell. In a multi-cell deployment, S-CDMA enables a truefrequency reuse of one. With S-CDMA, there is no need for frequencyplanning, and spectral efficiency is maximized. With a frequency reuseof one, the total system spectral efficiency is equal to the modulationfactor of a given cell. Comparing S-CDMA to a single carrier TDMAapproach, with a typical frequency reuse of 4, TDMA systems must achievemuch higher modulation factors in order to compete in terms of overallsystem spectral efficiency. Assuming no sectorization and a frequencyreuse of one, S-CDMA systems can achieve system spectral efficienciesfrom 1 to 6 bps/Hz, with improvements being possible with SDMA.

While frequency reuse of one is theoretically possible for DS-CDMA, thetrue allowable reuse of a specific deployment is dependent on thepropagation environment (path loss) and user distribution. For mobilecellular systems, it has been shown that realistic reuse factors rangefrom 0.3 up to 0.7 for A-CDMA: factors that are still much higher thanfor TDMA systems. In a S-CDMA system, in-cell interference is mitigatedby the orthogonal nature of the S-CDMA, implying that the dominantinterference results from adjacent cells. For the fixed environmentsusing S-CDMA, true frequency reuse of one can be achieved for mostdeployments using directional SS antennas and up and downstream powercontrol to mitigate levels of adjacent cell interference. In a S-CDMAenvironment, true frequency reuse of one implies that a cell iscode-limited, even in the presence of adjacent cell interference.

For sectorized deployments with S-CDMA, a frequency reuse of two isrequired to mitigate the interference contributed by users on sectorboundaries. In light of this reuse issue, it is preferred to use SDMAwith adaptive beamforming rather than sectorization to improve cellcapacity.

Since spectral efficiency translates directly into cost, the possibilityof a frequency reuse of one is an important consideration.

The use of SDMA in conjunction with S-CDMA offers the ability todramatically increase system capacity and spectral efficiency. SDMA usesan antenna array at the BS 11 to spatially isolate same code SSs 10 inthe cell. The number of times that a code may be reused within the samecell is dependent upon the number of antenna elements in the array, thearray geometry, the distribution of users in the cell, the stability ofthe channel, and the available processing power. Theoretically, in theabsence of noise, with an M element antenna array it is possible toreuse each code sequence M times, thereby increasing system capacity bya factor of M. In practice, the code reuse is slightly less than M dueto implementation loss, frequency selective multipath fading, andreceiver noise. Regardless, significant capacity gains are achievablewith SDMA. With appropriate array geometry and careful grouping of userssharing CDMA codes, it is possible to achieve a code reuse of 0.9M orbetter.

In an actual deployment the number of antenna elements is limited by theavailable processing power, the physical tower constraints, and systemcost (e.g. the number of additional RF front ends (RFFEs)). Selectedarray sizes vary depending upon the required capacity of the given cellon a cell-by-cell basis. The Table shown in FIG. 5 illustrates theachievable aggregate capacity and modulation factor with typical arraysizes, assuming a code reuse equal to the number of antenna elements.The aggregate capacity is defined as the total data rate of the BS 11.Modulation factors exceeding 56 bps/Hz are achievable with 64 QAM and asixteen-element antenna array. It should be noted that while SDMAincreases the capacity of cell, it does not increase the peak data rateto a given SS 10.

The PHY system disclosed herein is very flexible. Using narrowbandS-CDMA channels, the PHY system can adapt to frequency allocation,easily handling non-contiguous frequency allocations. The datamultiplexing scheme allows great flexibility in servicing trafficasymmetry and support of traffic patterns created by higher-layerprotocols such as TCP.

Deployments using the disclosed PHY are also very scalable. When trafficdemands increase, new frequency allocation can be used. This involvesadding additional FDMA channels, which may or may not be contiguous withthe original allocation. Without additional frequency allocation, cellcapacity can be increased using an adaptive antenna array and SDMA.

The high spectral efficiency of the disclosed waveform leads to costbenefits. High spectral efficiency implies less frequency bandwidth isrequired to provide a certain amount of capacity.

Using a symmetric waveform (i.e., a waveform that is the same in theupstream and downstream directions) is a cost saving feature, allowingthe use of common baseband hardware in the SS 10 and the BS 11. The useof CDMA technology also aids in cost reduction, as some CDMA technologydeveloped for mobile cellular applications may be applicable to gaineconomies of scale.

As a spread spectrum signal, the preferred waveform offers inherentrobustness to interference sources. Interference sources are reduced bythe spreading factor, which ranges from 1 to 128 (interferencesuppression of 0 to 21 dB.) At the SS 10, equalization furthersuppresses narrowband jammers by adaptively placing spectral nulls atthe jammer frequency. Additional robustness to interference is achievedby the directionality of the SS antennas, since off-boresightinterference sources are attenuated by the antenna pattern in thecorresponding direction. At the BS 11, the antenna array used toimplement SDMA offers the additional benefit of adaptively steeringnulls towards unwanted interference sources.

The presently preferred waveform exhibits several properties that makeit robust to channel impairments. The use of spread spectrum makes thewaveform robust to frequency selective fading channels through theinherent suppression of inter-chip interference. Further suppression ofinter-chip interference is provided by equalization at the SS 10. Thewaveform is also robust to flat fading channel impairments. The adaptivechannel coding provides several dB of coding gain. The antenna arrayused to implement SDMA also functions as a diversity combiner. Assumingindependent fading on each antenna element, diversity gains of M areachieved, where M is equal to the number of antenna elements in thearray. Finally, since the S-CDMA system is code-limited rather thaninterference limited the system may run with a large amount of fademargin. Even without equalization or diversity, fade margins on theorder of 10 dB are possible. Therefore, multipath fades of 10 dB or lessdo not increase the BER beyond the required level.

The adaptive modulation also provides some robustness to radioimpairments. For receivers with larger phase noise, the QPSK modulationoffers more tolerance to receiver phase noise and filter group delay.The adaptive equalizer at the SS 10 reduces the impact of linear radioimpairments. Finally, the use of clipping to reduce the peak-to-averagepower ratio of the transmitter signal helps to avoid amplifiersaturation, for a given average power output.

An important distinction between the presently preferred embodiment anda number of other CDMA approaches is the use of a synchronous upstream,which allows the frequency reuse of one. Due to some similarity withmobile cellular standards, cost savings are possible using existing,low-cost CDMA components and test equipment.

The presently preferred PHY is quite different from cable modem and xDSLindustry standards, as well as existing IEEE 802.11 standards. However,with a spreading factor of one chip/symbol, the PHY supports asingle-carrier QAM waveform similar to DOCSIS 1.1 and IEEE 802.16.1draft PHY (see “Data-Over-Cable Service Interface Specifications: RadioFrequency Interface Specification”, SP-RF1v1.1-I05-000714, and IEEE802.16.1-00/01r4, “Air Interface for Fixed Broadband Wireless AccessSystems”, September 2000.

The presently preferred PHY technique provides an optimum choice forIEEE 802.16.3 and for other applications. An important aspect of the PHYis its spectral efficiency, as this translates directly to cost measuredin cost per line or cost per carried bit for FWA systems. With afrequency reuse of one and efficient support of SDMA for increasedspectral efficiency, the combination of S-CDMA with FDMA is an optimumtechnology for the fixed wireless access market.

Benefits of the presently preferred PHY system include:

High spectral efficiency (1–6 bps/Hz system-wide), even without SDMA;

Compatibility with smart antennas (SDMA), with system-wide spectralefficiency exceeding 20 bps/Hz possible; and

A frequency reuse of one possible (increased spectral efficiency and nofrequency planning).

The use of S-CDMA provides robustness to channel impairments (e.g.multipath fading): robustness to co-channel interference (allowsfrequency reuse of one); and security from eavesdropping.

Also provided is bandwidth flexibility and efficiency support of QoSrequirements, flexibility to support any frequency allocation using acombination of narrowband SC-DMA combined with FDMA, while adaptivecoding and modulation yield robustness to channel impairments andtraffic asymmetries.

The use of these teachings also enables one to leverage mobile cellulartechnology for reduced cost and rapid technology development and test.Furthermore, cost savings are realized using the symmetric waveform andidentical SS 10 and BS 11 hardware.

Having thus described the overall PHY system, a discussion will now beprovided in greater detail of an aspect thereof that is particularlypertinent to the teachings of this invention.

Most cellular and fixed wireless access air-interfaces use either CDMA,FDMA or TDMA to provide logical channels to users. The instant inventionemploys the use of both CDMA and FDMA to provide a variable bandwidthwaveform with multiple bonded transmitters and receivers that are agilein both frequency and PN code to permit a variable bandwidth andvariable rate multiple access system.

There are at least two important aspects of this invention. The first isthe use of both CDMA and FDMA together to provide a improvedconcentration efficiency by making a larger pool of bandwidth availableto each user. The second aspect enables channel bonding across both codespace and frequency space, thus making the system capable of operatingin a variable (not necessarily contiguous) bandwidth and at a finelyvariable rate.

As is shown in FIG. 6, by providing multiple modulators 26A, 26B, . . ., 26 n and demodulators 34A, 34B, . . . , 34n to each subscriber unit10, and allowing multiple links to be established between eachsubscriber and the base station 11 with data multiplexed (multiplexer25) across the multiple modulators 26A, 26B, . . . , 26 n at thetransmitter 20 and demultiplexed (demultiplexer 33) from the multipledemodulators 34A, 34B, . . . , 34 n at the receiver 30, a much moreflexible variable rate system is created. Modulators 26A, 26B, . . . ,26 n operate at sub-channel frequencies fA, fB, . . . , fn with PN codesPN_A, PN_B, . . . , PN_n, respectively, where n is an integer (e.g., 4,5, 6, 7, etc.), and the demodulators 34A, 34B, . . . , 34 n operateaccordingly. In the example mentioned above, where it was desired tooperate the link at 1050 kbps, a 1024 kbps channel may be bonded with a32 kbps channel to achieve a 1056 kbps channel, thus producing verylittle wasted bandwidth as compared to the prior art approach. So longas the demultiplexing pattern in the receiver 30 matches themultiplexing pattern of the transmitter 20, then the link appears toeffectively run at the 1056 kbps rate, and the fact that the data wasactually transmitted through two separate channels is transparent to theuser.

As was stated above, channel bonding, namely transmitting and receivingon multiple links in parallel to achieve additional rates, is known inCDMA systems. If, however, each modulator 26 and demodulator 34 is bothfrequency and code agile, then both dimensions can be utilized toprovide effective rate granularity. Thus, if a subscriber unit 10 has,for example, N modulators 26 and N demodulators 34, which are eachcapable of communicating at rates that are power of two multiples of abasic rate on a variety of frequency subchannels, then a tremendousamount of flexibility is provided in achieving rates between thoseachievable by a single channel. If, for example, N equals seven, andeach channel can achieve power of two multiples of 32 kbps from 32 kbpsto 2.048 Mbps, then these bonded channels can achieve any integer (notjust power of two integer) multiple of 32 kbps between 32 kbps and 4.096Mbps. If one frequency subchannel is being heavily utilized by othertraffic within the cell, then the BS 11 has the flexibility ofreassigning some of the channels to other less-used frequencysubchannels. Clearly the granularity of the variable rate effectivechannel is dramatically improved by bonding channels in accordance withthese teachings.

A related advantage of two-dimensional channel bonding (alluded toabove) is that the use of both dimensions provides a larger pool ofchannels to draw from, thus improving the ability to gain statisticalconcentration. For example, if a CDMA system has 4.096 Mbps of aggregatecapacity which may be allocated to X users simultaneously at rates of4.096/X Mbps each, then by making the modulators 26 and demodulators 34capable of tuning to any one of four frequency subchannels, the actualuseable pool of bandwidth is four times the 4.096 Mbps bandwidth of anyone channel, and 4X users can be supported simultaneously at rates of4.096/X Mbps. This implies that, due to an improved erlang efficiency,if Y users can statistically share the X channels in one frequencysubchannel, then more than 4Y users can share the 4X channels in fourfrequency subchannels.

Another advantage of adding frequency agility to a PN-code agilemodulator 26 and demodulator 34 is that it permits the system to haveflexibility in its consumed bandwidth. For example, a system that canoperate only with 14 MHz wide channels cannot be used if the bandwidthallocated to the system is only 3.5 MHz. On the other hand, if a systemuses CDMA/FDMA with channel bonding, then both the BS 11 and the SSs 10have a bank of receivers that can each independently be tuned to one ofa variety of frequencies, in addition to one of a variety of PN codes.If the bandwidth of any one subchannel is, for example 3.5 MHz, then bytuning some of the modulators and demodulators to each 3.5 MHz slotwithin a 14 MHz allocation, the bandwidth can be consumed efficiently.Thus a CDMA/FDMA system with four 3.5 MHz subchannels can operate in a14 MHz channel, but a 14 MHz bandwidth CDMA system can not operate in a3.5 MHz channel. Furthermore, even though a 10.5 MHz bandwidth pure CDMAsystem and a CDMA/FDMA system with three 3.5 MHz subchannels occupy thesame bandwidth and provide approximately the same throughput when fullyloaded, the CDMA/FDMA hybrid system is far more flexible. For example,if a 14 MHz frequency allocation is divided into four 3.5 MHzsubchannels (labeled A, B, C and D) and subchannel C is allocated toanother system, then a 10.5 MHz bandwidth pure CDMA system could notoperate. In contrast, a CDMA/FDMA system could simply use subchannels A,B and D, leaving subchannel C to the other system. The ability to usenon-contiguous subchannels provides operators a unique flexibility thatcan be very useful when attempting to add a new service to a band offrequency where some of the frequency subchannels have previously beenallocated to other systems.

The use of CDMA/FDMA permits the system to occupy a variable bandwidth(either contiguous or noncontiguous). For example, a CDMA/FDMA hybridsystem with four 3.5 MHz subchannels can easily operate as a 3.5 MHzsystem, a 7 MHz system, a 10.5 MHz system or a 14 MHz system. Again,since these do not need to be contiguous, an additional degree offlexibility is achieved over a pure CDMA system.

Another aspect to the FDMA augmentation of the CDMA system is that ifthere is a noisy subband within, for example, a 14 MHz band allocated tothe system, then that subband can be adaptively avoided. If the BS 11and SS 10 are capable of detecting link quality, then they can reducethe capacity of a particular 3.5 MHz channel and place the bulk of thetraffic in the less-noisy channels. This approach has the benefits of anOFDM approach without much of the complexity of OFDM.

A related benefit of this approach is that the traffic may be spreadevenly across the bands if they are all equally “clean”. This has theadvantage that in the forward channel, where the power allocated to eachchannel reduces as users are added, the system can maximize the powerallocated to each channel by keeping the number of active users as lowas possible in each channel.

In accordance with these teachings, by bonding N modulators 26 anddemodulators 34 together, and multiplexing the data to the modulators 26and demultiplexing it at the demodulators 34, an effective channel iscreated that operates with a fine granularity of achievable data rates.Furthermore, the use of N modulators 26 and demodulators 34, which caneach run at a unique rate on a unique code and a unique frequencysubchannel, permits the link to exhibit the characteristics of occupyinga flexible channel bandwidth while providing a great deal of flexibilityin setting the rate of the link. The end result is an efficientutilization of the bandwidth resource.

While the invention has been particularly shown and described withrespect to preferred embodiments thereof, it will be understood by thoseskilled in the art that changes in form and details may be made thereinwithout departing from the scope and spirit of the invention.

1. A method for operating a communication system, comprising steps of:defining the system as a combined Code Division Multiple Access CDMA andFrequency Division Multiple Access FDMA system, wherein CDMA is usedwithin each FDMA sub-channel, and a plurality of PN code channels areavailable in each of the FDMA sub-channels; using at least one basestation and a plurality of subscriber stations, where the at least onebase station and each of the plurality of subscriber stations have aplurality of frequency agile and PN code agile data modulators anddemodulators, and wherein each of the frequency agile and PN code agiledata modulators and demodulators can be selectively tuned to separate PNcode channels operating within the FDMA sub-channels; determiningcurrent transmission requirements based, at least in part, on operatingconditions; and selecting a number of FDMA sub-channels sufficient tomeet current transmission requirements, wherein the system operates as avariable bandwidth system as the number of FDMA sub-channels varies dueto transmission requirements.
 2. A method as in claim 1, wherein the useof both CDMA and FDMA together provides an improved concentrationefficiency by making a larger pool of bandwidth available to each user.3. A method as in claim 1, further comprising: performing channelbonding in both code and frequency space by bonding PN code channelsfrom separate FDMA sub-channels.
 4. A synchronous Code Division MultipleAccess CDMA and Frequency Division Multiple Access FDMA communicationssystem, comprising: a plurality of FDMA sub-channels, wherein CDMA isused within each of the FDMA sub-channels; a base site comprising atransmitter for transmitting a waveform and further comprising aplurality of frequency agile and PN code agile data modulators having anoutput coupled to a radio channel, wherein the transmitter is operableto perform channel bonding in both frequency space and code space bybonding PN code channels from different FDMA sub-channels, forming aneffective channel; and a subscriber unit comprising a receiver forreceiving the transmitted waveform from the radio channel and furthercomprising a plurality of frequency agile and PN code agile datademodulators, wherein the subscriber unit is operable to receive datatransmitted in the effective channel formed by bonding PN code channelsfrom different FDMA sub-channels.
 5. A CDMA and FDMA communicationssystem as in claim 4, wherein there are N modulators and N demodulatorseach operable for communicating at data rates that are power of twomultiples of a basic rate on a plurality of frequency subchannels withina channel.
 6. A CDMA and FDMA communications system as in claim 5,wherein said N modulators and N demodulators operate with power of twomultiples of the basic rate from a minimum rate to a maximum rate at agranularity that is an integer multiple of the basic rate.
 7. A CDMA andFDMA communications system as in claim 4, wherein statisticalconcentration is achieved when the system has Y Mbps of aggregatecapacity allocatable to X users simultaneously at rates Y/X Mbps each,and by tuning said modulators and demodulators to any one of Z frequencysubchannels, the useable bandwidth is Z times the Y Mbps bandwidth ofany one channel, and Z*X users are supported simultaneously at rates ofY/X Mbps.
 8. A CDMA and FDMA communications system as in claim 4,wherein a bandwidth of any one subchannel is X MHz, and at least some ofsaid plurality of modulators and demodulators are tuned to differentones of contiguous or non-contiguous X MHz sub-channels within a Y MHzchannel, where Y>X.
 9. A CDMA and FDMA communications system as in claim8, wherein X=3.5 and Y=14.
 10. A CDMA and FDMA communications system asin claim 4, wherein input data to said plurality of modulators is apunctured convolutional code.
 11. A CDMA and FDMA communications systemas in claim 4, wherein input data to said plurality of modulators is arate 1/2, constraint length 7 code that is punctured to increase therate.
 12. A CDMA and FDMA communications system as in claim 11, whereinthe puncturing rate is made adaptive to mitigate fading conditions. 13.A CDMA and FDMA communications system as in claim 11, wherein saidoutput of said modulators is coupled to said radio channel through anend-to-end raised-cosine Nyquist pulse shape filter.
 14. A method foroperating a communication system, comprising steps of: defining thesystem as a combined Code Division Multiple Access CDMA and FrequencyDivision Multiple Access system; and using a variable bandwidth waveformwith multiple bonded transmitters and receivers that are each agile inboth frequency and code to provide a variable bandwidth and variablerate multiple access system, wherein channel bonding across both codespace and frequency space enable the system to operate in at least oneof a variable, contiguous or non-contiguous bandwidth at a finelyvariable rate, the channel bonding across both code space and frequencyspace bonding CDMA channels from different FDMA sub-bands together. 15.The method of claim 14 wherein channel bonding across frequency spacebonds FDMA sub-bands together.
 16. A method for operating acommunications system, comprising steps of: defining the system as acombined Code Division and Frequency Division Multiple Access FDMAsystem, wherein CDMA is used in each FDMA sub-band; using a variablebandwidth waveform with multiple bonded transmitters and receivers thatare each agile in both frequency and code to provide a variablebandwidth and variable rate multiple access system, wherein channelbonding occurs across both code space and frequency space, the channelbonding across both code space and frequency space enabling thecommunication system to operate at a finely variable rate in a variablebandwidth comprised of contiguous FDMA sub-bands, wherein the number ofFDMA sub-bands used varies in dependence on operating conditions.
 17. Amethod for operating a communication system, comprising steps of:defining the system as a combined Code Division and Frequency DivisionMultiple Access FDMA system, wherein CDMA is used in each FDMA sub-band;using a variable bandwidth waveform with multiple bonded transmittersand receivers that are each agile in both frequency and code to providea variable bandwidth and variable rate multiple access system, whereinchannel bonding occurs across both code space and frequency space, thechannel bonding across both code space and frequency space and frequencyspace enabling the system to operate at a finely variable rate in avariable bandwidth comprised of non-contiguous FDMA sub-bands, whereinthe number of FDMA sub-bands used varies in dependence on operatingconditions.